Some known methods for creating simulated wood appearance on plastics have employed a compressed or extruded mixture of wood particles and polymer to form an object. Including real wood-based material is problematic in terms of material and manufacturing cost, material compatibility and the properties of the final product.
There have been some attempts to use co-extrusion to make plastic articles that have a wood grain appearance. One known method uses a feed screw within an oversized barrel to advance plastic including two different colors, so that a significant gap in between the barrel and the screw causes backflow of the plastic behind the screw threads to promote flow surges to ultimately allow colored “swirl” patterns. This technique has a number of disadvantages. It requires elevated screw rotation speed to advance the plastic quickly enough so that the color pigments do not completely disperse into the plastic which would undesirably create a uniformly coloured material. This high dependence and sensitivity to screw speed also makes it difficult to control shear conditions and reproduce the process to manufacture a consistent end product. The reliance on fast screw rotation also obliges that the temperature be reduced, again to avoid completely melting and dispersing the pigments throughout the material. In addition, specially designed equipment (barrel and screw) must be used to allow the required gap to promote the backflow surges. Premature wear on the screw or premature wear on the barrel will diminish this effect to the point where “swirls” are no longer apparent; not enough back pressure on the die will also reduce the swirl effect or to the point where linear lines become apparent which is not the desired effect. The technique tends to produce a pattern of linear streaks, which can resemble some wood types but is not always the desired wood grain effect.
There is indeed a need for a technology that can overcome at least some of the disadvantages of what is known in this filed.